为什么选定值不能反映在带有颤动的showModalBottomSheet中？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Modal Bottom Sheet Example')),
        body: ModalBottomSheetExample(),
      ),
    );
  }
}

class ModalBottomSheetExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: ElevatedButton(
        onPressed: () {
          showModalBottomSheet(
            context: context,
            builder: (BuildContext context) {
              return StatefulBuilder(
                builder: (BuildContext context, StateSetter setState) {
                  String selectedValue = 'Initial Value';

                  return Column(
                    mainAxisSize: MainAxisSize.min,
                    children: <Widget>[
                      DropdownButton<String>(
                        value: selectedValue,
                        onChanged: (String? newValue) {
                          setState(() {
                            selectedValue = newValue!;
                          });
                        },
                        items: <String>['Option 1', 'Option 2', 'Option 3']
                            .map<DropdownMenuItem<String>>((String value) {
                          return DropdownMenuItem<String>(
                            value: value,
                            child: Text(value),
                          );
                        }).toList(),
                      ),
                      Text('Selected Value: $selectedValue'),
                    ],
                  );
                },
              );
            },
          );
        },
        child: Text('Show Modal Bottom Sheet'),
      ),
    );
  }
}